For this, I am assuming that, in Infinity War, the shield is destroyed and it will appear as though Steve Rogers has died (as foreshadowed in Age of Ultron.)

The new series begins with Captain America: Fourth Reich, featuring the "return" of Captain America/Steve Rogers. The reality of the situation is that he has been transported into and revived within the realm of the Tesseract/Cosmic Cube. It is established that here, where the Red Skull has been residing for decades, he has created a reality where WW2 continued, and now the USA is the last remaining democracy. When Steve arrives, the year is 1963 and JFK has just been assasinated by HYDRA.

Basically, Rogers teams up with Namor, Agent Carter and Howard Stark (who has created a suit which allows him to be The Human Torch) and they begin to stage an invasion of the Fourth Reich.

At some point, Bucky and Falcon become aware of the situation and team up to also dive into the Cube in an effort to rescue Rogers.

I would love to see a series of films, each one going further back in time within the alternate reality of the Cube. The first one, as mentioned is around 1960 and establishes the situation. From there, I would also like to see one set during the goldrush/wild west era and finally one set during the colonial/Revolutionary War. Basically any iconic time period of American history would be awesome. The films do not intersect with the rest of the MCU beyond the fact that the characters originally came from there. For all intents and purposes, these characters are all "dead" in the actual MCU, until such a time as they find out how to escape the Cube.